Radiography has been employed for many years as a medical diagnostic tool. A subject to be studied is placed between a x-ray radiation source and a detection system which typically includes an intensifying screen and a suitable photographic film. Intensifying screens have been employed in the art as a conversion device wherein x-ray radiation is converted to lower energy radiation such as visible radiation. Photographic film captures the image emitted by the intensifying screen and with subsequent development of the photographic film an image is generated which represents the variations in absorption of x-ray radiation as it passed through the subject.
Subject dosage and image quality are typically directly related for a given film/screen combination. There is an ongoing need to improve the image quality without increasing overall dosage for the subject. The dilemma has been advanced substantially by the use of both sensitizing and acutance dyes within the photographic element.
Sensitizing dyes are known in the art as a means for increasing the sensitivity of a silver halide emulsion to a specific band of wavelengths. A myriad of dyes have been presented to the art field as exemplified in Research Disclosure, No 308, December, 1989, Item 308119. In the field of radiography the spectral response of the silver halide emulsion preferably corresponds to the blue or green emission of the intensifying screen.
Acutance dyes have been presented in the art as a means of greatly improving image quality with subsequent minor increase in subject dose. This improved image quality is accomplished by decreasing the amount of light which scatters within the emulsion and more importantly by decreasing the amount of cross-over. Cross-over typically refers to screen emission which passes through, and is scattered by, the closest emulsion and the substrate and is subsequently captured by the emulsion on the opposite side of the support. The scattering of the emission as it passes through the support decreases the resolution of the resulting image. Pyrazolone dyes have shown great utility as crossover dyes as exemplified in U.S. Pat. Nos. 4,900,652; 4,948,718; 4,803,150; 4,855,221; 4,940,654 and 4,857,446.
In general, acutance or cross-over dyes compete with the silver halide grains for available emission from the intensifying screen resulting in a loss of overall photographic speed. The practitioner of the art is therefore forced to reach a balance between the photographic speed and resolution for a particular application.
Recent advances in the art include the use of intensifying screens which are comprised of phosphors which emit in the ultraviolet. One advantage of systems utilizing these phosphors is the inherent UV absorption of the photographic supports typically employed in the art. Cross-over is reduced substantially without the use of dyes and, in fact, one practicing the art would prefer to exclude acutance dyes in a system utilizing UV emitting screens. The resolution obtained with UV intensifying screens typically far exceeds the prior art techniques which employ acutance dyes and conventional intensifying screens. Further improvements with dyes are not expected to be warranted and, in fact, would be considered to be detrimental in light of the expected loss in system speed.
Contrary to these teachings from the art is a dye family which is well known in the art as an acutance dye, yet when utilized within the teachings provided herein has the unexpected property of increasing the system speed in the ultraviolet. Therefore, instead of decreasing system speed with a corresponding improved resolution as observed with blue and green emitting phosphors, films containing these dyes actually demonstrate an increased speed at a comparable resolution. An increase in speed is observed in spite of the lack of ultraviolet absorption by the dye.